ListNode a = new ListNode(-1);ListNode p = a;p.next = nexListNode;p = p.next;

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
 public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
 ListNode point1 = list1;
 ListNode point2 = list2;
 ListNode head = new ListNode(-1);
 ListNode p = head;
  
 if (point1 == null && point2 == null) {
 return null;
 }
 while (point1 != null && point2 != null) {
 if (point1.val < point2.val) {
 p.next = point1;
 point1 = point1.next;
 }
 else {
 p.next = point2;
 point2 = point2.next;
 }
 p = p.next;
 }
  
 // 拼接剩下的
 if (point2 != null) {
 p.next = point2;
 }
 if (point1 != null) {
 p.next = point1;
 }
  
 // 去掉临时头
 head = head.next;
  
 return head;
 }
}